High Frequency Or Multifrequency Resistivity Tool
A system and method for determining material resistivity. A current of one or more frequencies is generated using a circuitry. The one or more frequencies is directed using one or more electrodes into a surround-ing material. The resistivity of the material surrounding the electrodes and a drilling tool is measured by detecting the current at the one or more fre-quencies.
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This invention relates to the field of well logging. More particularly, the invention relates to a method and apparatus for improved injection of current into a formation and reduction of the effect of standoff distance from the formation during resistivity measurements.DESCRIPTION OF THE RELATED ART
Resistivity logging, which measures the electrical resistivity of earth formations surrounding a borehole, is a commonly used technique for formation evaluation. In general, porous formations having high resistivity are filled with hydrocarbons, while porous formations having low resistivity are water saturated. One technique used to measure formation resistivity is electrode logging. This resistivity measurement technique uses various arrangements of electrodes on a logging device to generate and measure electrical currents and/or potentials from which one determines formation resistivity.
Various formation resistivity logging tools and techniques exist to perform resistivity logging. One of such tools that may be used for resistivity logging is a laterolog tool. A laterolog tool has lateral electrodes and is described in U.S. Pat. No. 3,305,771 to Arps; U.S. Pat. No. 5,235,285 to Clark et al.; and S. Bonner et al., “A New Generation of Electrode Resistivity Measurements for Formation Evaluation While Drilling”, SPWLA, June 1994. All of the above documents are incorporated herein by reference. The electrodes are mounted using insulation on the electrically conductive body of the LWD tool. The electrodes may be mounted on the tool collar, stabilizer blades, or some other part of the tool body.
During formation resistivity measurements, a voltage difference is created between two sections of the electrically conductive drill collar. The tool may contain a device to generate a voltage and a device to measure the resulting current through the one or more electrodes. The electrode to measure current may be a ring electrode or button electrode. The ring electrode comprises a metal band around the tool while the button electrode comprises a metal disc mounted on the tool collar. Both ring and button electrodes are electrically isolated from the tool. The collar surrounding the ring or button electrode acts as a guard electrode to focus more of the electrode current into/out of the formation.
During a resistivity logging operation, an impedance layer may develop on the tool collar and/or the surface of an electrode and may affect the flow of current between the electrode and borehole fluid or mud. The value of contact impedance depends on a number of factors (electrode potential, temperature, electrode material and roughness, exposure time, pH, fluid salinity, and frequency) and is highly variable.
The surface impedance layer can cause the current to be different from what it would have been in a perfect tool without surface impedance. When current flows into or out of an electrode or into or out of the collar through a surface impedance layer, a voltage drop is produced and the potential immediately outside the metal becomes different from the potential inside the metal. A particularly damaging effect occurs whenever two nearby electrodes (i.e. an electrode and the tool collar) have different surface impedances. When this situation occurs, the potential immediately outside the electrode will be different from that immediately outside of the tool collar. This difference causes a current to flow between the electrode and the tool collar, which affects the current flowing through the formation. The smaller the mud resistance, the larger the effect of the surface impedance.
Another bad effect of the surface impedance layer is to reduce the effectiveness of the tool collar as a guard electrode, reducing clarity of the contrast measurement of thin formation layers of different conductivity, effectively reducing the tool's vertical and azimuthal resolution.
One type of laterolog tool that may minimize the effect of surface impedance employs separate voltage monitoring electrodes to sense the voltage of the mud near the surface of the tool. The monitor electrodes emit essentially no current and so are unaffected by surface impedance. Focusing is achieved by means of a feedback loop that adjusts the bucking or survey current to maintain monitoring electrodes at the same voltage.
The diameter of a typical button electrode in a resistivity logging tool is about one inch but with two surrounding monitor electrodes, the diameter of the electrode assembly may well exceed two inches. Since the electrode assembly is typically placed on the tool collar, stabilizer blades, or some other part of the tool body, the electrode assembly becomes extremely vulnerable in a drilling environment. Accordingly, using separate voltage monitoring electrodes in the resistivity logging tool to minimize the effect of surface impedance on resistivity measurements makes the tool more complex and reduces the reliability/survivability of the tool in an LWD environment.SUMMARY
In one respect, disclosed is a method for resistivity logging, the method comprising: generating a current, the current oscillating at one or more frequencies, at least one of the frequencies being above a critical frequency; directing the current through a surrounding material; and detecting the current and, in response thereto, determining a resistivity of the surrounding material at the one or more frequencies.
In another respect, disclosed is an apparatus for resistivity logging, the apparatus comprising: a current generating device, the device being operable to generate a current oscillating at one or more frequencies, at least one of the frequencies being above a critical frequency; one or more electrodes coupled to the generating device, the electrodes being adapted to direct current through. a surrounding material; and an analysis device coupled to the one or more electrodes, the analysis device being operable to detect the current and determine a resistivity of the surrounding material.
Numerous additional embodiments are also possible.
Other objects and advantages of the invention may become apparent upon reading the detailed description and upon reference to the accompanying drawings.
While the invention is subject to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and the accompanying detailed description. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular embodiment. This disclosure is instead intended to cover all modifications, equivalents, and alternatives falling within the scope of the present invention as defined by the appended claims.DETAILED DESCRIPTION
One or more embodiments of the invention are described below. It should be noted that these and any other embodiments are exemplary and are intended to be illustrative of the invention rather than limiting. While the invention is widely applicable to different types of systems, it is impossible to include all of the possible embodiments and contexts of the invention in this disclosure. Upon reading this disclosure, many alternative embodiments of the present invention will be apparent to persons of ordinary skill in the art.
Examples of such electrodes on a tool used for resistivity measurements are described in S. Bonner et al., “A New Generation of Electrode Resistivity Measurements for Formation Evaluation While Drilling”. which is incorporated herein by reference. As the tool rotates and advances into the earth formation, formation resistivity may be determined by studying the current flowing through one or more such electrodes. The variation of the resistivity in different directions and positions may be used to determine the composition of the formation at those directions and positions.
Another resistivity imaging LWD tool and field test results using the tool are described in Ritter et al, “High Resolution Visualization of Near Wellbore Geology Using While-Drilling Electrical Images”, SPWLA 2004, paper PP, which is also incorporated herein by reference.
Circuitry 225 is operable to generate a voltage and consequently current that can flow from/to electrode 215 to/from the body of drilling tool 210. By doing so, the resistivity of the material surrounding electrode 215 and tool 210 may be measured and the material's composition may be estimated. Circuitry 225 may contain, among other components, operational amplifiers, resistors, transformers, and voltage generators configured to output DC and AC voltages at different frequencies, amplitudes, and phases. Other types of electrodes—as well as additional electrodes—having the same functionality as the electrode shown in
In one embodiment, circuitry 225 is operable to generate currents at high frequencies as well as multifrequency currents to overcome problems that may occur from increased impedance on the surface of the electrode. Such increase in impedance may occur over time, for example, due to the built up of residue on the surface of electrode 215 and drilling tool 210. U.S. Pat. No. 6,373,254 to Dion, which is incorporated herein by reference, describes some of these problems. The increase in the surface impedance as well as the differences in surface impedance that may build up degrades the current focusing effect of the electrode and leads to artifacts and noise in the detected responses.
In one embodiment, electrode response accuracy may be improved by using alternating current at a number of different high frequencies for resistivity measurement. Due to the capacitive nature of the surface impedance, higher current frequencies can be affected less (experience reduced impedance) compared to lower frequencies. In addition, by taking measurements at multiple frequencies, the behavior of the surface impedance layer can be studied and its effects minimized.
In one embodiment, salt water having a resistivity of 0.1 ohm-meter is used as conductive fluid 330. In certain cases, the rods that are used as electrodes are cleaned with an abrasive pad to ensure that no surface impedance layer exists in the beginning of the experiment. In one session, the magnitude and phase of the impedance of the system is measured over time of about two hours at different frequencies (in the range from 100 Hz to 100 KHz). The data is then fit to the RC-model shown in
As shown in graphs 410 and 415 of
Graph 510 of
Standoff 650 represents the distance between electrode 635 surface and the sidewall of the borehole (ground electrode 615). In one embodiment, to obtain accurate resistivity data, the standoff distance between the electrode surface and the sidewall is be kept as small as possible. A small standoff distance, however, makes the electrodes more vulnerable to damage caused by the sidewall of the borehole in a LWD drilling environment. To protect the electrodes from direct mechanical impact, the electrode surfaces may be slightly recessed below the surface of the tool collar/stabilizer/blade 620. Thus, the rate and degree of cleaning the electrode surface and tool collar/stabilizer blade surface will be different. This results in non-identical build up of impedance layers on electrode 635 surface and tool collar/stabilizer/blade 320. This, in turn, may lead to creation of leakage currents and noise artifacts in the resistivity response of the electrode.
Noise artifacts may become pronounced while logging resistive beds, but are also visible in conductive beds, especially while crossing high-resistivity boundaries.
In one embodiment, conductive fluid 640 may be salt water with a resistivity of 0.1 ohm-meter. The inner wall of the barrel is covered with a steel sheet (ground electrode 615) to form a wide-area ground electrode. When AC voltage is applied between the tool collar and the ground electrode, current flows through the salt water laterally to the collar surface. The current flow pattern in the test setup of
Graph 710 of
The method begins at 900 whereupon, at step 910, a current oscillating at one or more frequencies is generated. At least one of the frequencies of the current is above a critical frequency. At step 915, the current is directed through a surrounding material, and at step 920, the current is detected and a resistivity of the surrounding material is determined at the one or more frequencies. The method ends at 999.
Resistivity tools operating over a wide range of frequencies may yield additional useful information. Because AC current of different frequencies attenuates at different rates in the same conductive medium, meaningful interpretation of a multifrequency resistivity response may be derived.
In some embodiments, the resistivity logging is performed in multifrequency mode at a frequency above 1 KHz. In some embodiments of the invention, the frequency of AC current during resistivity logging may be 50 KHz or higher. A more accurate formation resistivity data may be determined based on multifrequency electrode response extrapolation and parameters for calibration of the response data as described above. Formation resistivity data may be further refined by independent determination of standoff distance.
Electrode response over a range of frequencies may be used to estimate the quality of collected resistivity log data and accept or reject it when the relative difference between the electrode responses collected at different frequencies will match or exceed some predefined value. In other words, electrode response at different frequencies are compared. The capacitive component of the surface impedance will be substantially negligible starting above a certain critical frequency, thereby enabling estimating of resistivity with a given accuracy. The response over a range of frequencies may also be used to gather resistivity data of the formation at different radii from the tool in order to create a lateral image of the earth formation.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the following claims be interpreted to embrace all such variations and modifications. By way of example, it is recognized that the disclosed method and apparatus for improved focusing of current and compensation for standoff distance effect during formation resistivity measurements may be performed during wireline well logging. In wireline well logging, a logging device suspended from a wireline cable is lowered into the borehole after the drill string has been removed. The logging device makes measurements while the cable is withdrawn.
Those of skill will appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Those of skill in the art may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.
The benefits and advantages that may be provided by the present invention have been described above with regard to specific embodiments. These benefits and advantages, and any elements or limitations that may cause them to occur or to become more pronounced are not to be construed as critical, required, or essential features of any or all of the claims. As used herein, the terms “comprises,” “comprising,” or any other variations thereof are intended to be interpreted as non-exclusively including the elements or limitations which follow those terms. Accordingly, a system, method, or other embodiment that comprises a set of elements is not limited to only those elements and may include other elements not expressly listed or inherent to the claimed embodiment.
While the present invention has been described with reference to particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the invention as detailed within the following claims.
1. A system comprising:
- a drilling tool for drilling a well, wherein the drilling tool comprises a conductive body;
- an electrode disposed on the conductive body; and
- an insulator configured to separate the conductive body from the electrode.
2. The system of claim 1, further comprising circuitry that connects the electrode to the conductive body.
3. The system of claim 2, wherein the circuitry being operable to detect a current and, in response, thereto, determine a resistivity of the conductive body at a plurality of different frequencies.
4. The system of claim 1, wherein the electrode being operable to generate a current oscillating at a plurality of different frequencies.
5. The system of claim 4, wherein the electrode is operable to measure resistivity.
6. The system of claim 1, wherein the electrode is a button electrode or ring electrodes.
7. The system of claim 1, further comprising an analysis device operable to detect one or more current characteristics, wherein the one or more current characteristics are chosen from the group including of an amplitude and a phase.
8. The system of claim 7, wherein the analysis device is operable to determine a difference between responses collected at different frequencies and to accept or reject collected resistivity log data.
9. The system of claim 7, further comprising a current generating device configured to provide an output AC voltage at a different frequency.
10. The system of claim 9, wherein the current generating device and the analysis device comprise components chosen from the group including of: resistors, operational amplifiers, and transformers.
11. A method comprising:
- disposing a drilling tool into a well, wherein the drilling tool comprises: a conductive body; an electrode disposed on the conductive body; and an insulator configured to separate the conductive body from the electrode. generating a current with the electrode;
- broadcasting the current into a surrounding material from the well that contains a conductive fluid; and
- detecting the current with the electrode.
12. The method of claim 11, further comprising oscillating the current at a plurality of different frequencies.
13. The method of claim 12, wherein at least one of the plurality of different frequencies is above a critical frequency.
14. The method of claim 11, further comprising determining a resistivity of the conductive body at a plurality of different frequencies.
15. The method of claim 14, further comprising forming a resistivity log from the resistivity at the plurality of different frequencies as log data.
16. The method of claim 15, further comprising determining a difference between the log data collected at different frequencies.
17. The method of claim 16, further comprising accepting or rejecting the log data.
18. The method of claim 11, wherein the detecting the current comprises detecting one or more current characteristics, wherein the current characteristics are chosen from the group including of: amplitude and phase.
19. The method of claim 11, wherein the broadcasting of the current comprises directing the current using one or more button or ring electrodes.
20. The method of claim 11, wherein the generating of the current comprises employing multiple voltage generators to output AC voltages each at different frequency.
Filed: Sep 24, 2019
Publication Date: Feb 13, 2020
Applicant: Halliburton Energy Services, Inc. (Houston, TX)
Inventors: Marian Morys (Downingtown, PA), Sergei Knizhnik (Exton, PA), William Francis Gallagher, III (Phoenixville, PA)
Application Number: 16/580,921